This invention relates to an internal combustion engine of a motor vehicle, equipped with a device for decreasing noise from the engine.
The origin of all important vehicle noises can be attributed to the production and application of the propulsive power or to the forward passage of the vehicle itself. As is known, the noise radiated from engines is a predominant source of noise. The engine noise is broadly classified into two kinds one of which is caused by the fact that the combustion noise generated in combustion chambers of the engine and mechanical noise generated in the rocker room pass through and are radiated out of the engine proper and the rocker cover of the engine. The other is caused by the fact that the combustion noise, due to crankshaft bearings, are conducted through the engine proper. Of these noises, the latter is converted into high frequency vibrations having frequencies of 1 KHz to 4 KHz while the noises are transmitted through a complicate engine structure, and thereafter the high frequence vibrations are radiated through the skirt portion of the crankcase, magnifying the vibrations since the skirt portion of the crankcase serves as a loudspeaker. Simultaneously, the high frequency vibrations cause the oil pan to vibrate and accordingly the oil pan becomes a new predominant noise source.
These noises radiated from the engine can be suppressed to a considerable extent by a sound-insulating cover for covering all or a part of the engine. However, this is not sufficient for the purpose of controlling engine noises. Because, the sound-insulating cover itself may considerably be vibrated due to the relationship between the natural frequency of and the vibration applied to the sound-insulating cover. Therefore, it is impossible to expect a great sound-insulating effect to such a sound-insulating cover.